The invention relates to methods of producing fludarabine or fludarabine phosphate. The invention also relates to intermediates useful in the production of fludarabine or fludarabine phosphate, as well as methods of producing such intermediates.
Fludarabine phosphate, also known as 9-.beta.-D-arabinofuranosyl-2-fluoroadenine-5'-phosphate, is a prodrug form of the anti-cancer agent, 9-.beta.-D-arabinofuranosyl-2-fluoroadenine, i.e., fludarabine or F-Ara-A. Accordingly, fludarabine phosphate is a chemotherapeutically effective form of the drug and is converted to the parent drug in vivo.
U.S. Pat. No. 4,210,745 discloses a method of synthesizing the anti-cancer agent, and U.S. Pat. No. 4,357,324 teaches phosphorylation of the cancer agent to yield the prodrug fludarabine phosphate. World patent application WO 91/08215 discloses a related method for the synthesis of fludarabine phosphate. In summary, fludarabine and fludarabine phosphate are commonly made by the following process:
(a) Acylation: 2,6-diaminopurine (also referred to as 2-aminoadenine) in a mixture of pyridine and a carboxylic acid anhydride is refluxed to yield a 2,6-diacylamidopurine, whereby the amino groups are protected with acyl groups; PA1 (b) Coupling: 2,3,5-tri-O-benzyl-1-O-p-nitrobenzoyl-D-arabinofuranose (TBNA) is converted to the corresponding chlorosugar, 2,3,5-tri-O-benzyl-1-.alpha.-chloro-D-arabinofuranose, which is then coupled with the 2,6-diacylamidopurine in a non-polar solvent such as ethylene dichloride and in the presence of a catalyst such as molecular sieves for several days until all of the chlorosugar is consumed or in the presence of a hydrochloric acid acceptor such as diisopropylethylamine, to yield the protected nucleoside 2,6-diacylamido-9-.beta.-D-(2',3',5'-tri-O-benzylarabinofuranosyl)purine; PA1 (c) Deacylation: The protected nucleoside of step (b) is refluxed with methanolic sodium methoxide to remove the acyl groups, yielding the O-protected nucleoside 2-amino-9-.beta.-D-(2',3',5'-tri-O-benzylarabinofuranosyl)adenine; PA1 (d) Diazotization/Fluorination: The O-protected nucleoside of step (c) undergoes diazotization and fluorination by reaction with sodium nitrite and fluoboric acid in a tetrahydrofuran-fluoboric acid (THF-HBF.sub.4) system to yield 2-fluoro-9-.beta.-D-(2',3',5'-tri-O-benzylarabinofuranosyl)adenine; PA1 (e) Debenzylation: The product from step (d) is treated with boron trichloride or with hydrogen and palladium chloride to remove the benzyl protecting groups, yielding 9-.beta.-D-arabinofuranosyl-2-fluoroadenine, the parent drug; or PA1 (f) Phosphorylation: The product from step (e) is mixed with phosphorous oxychloride in an alkyl phosphate such as triethylphosphate, or trimethylphosphate, followed by hydrolysis in water to yield the prodrug 9-.beta.-D-arabinofuranosyl-2-fluoroadenine-5'-phosphate or fludarabine phosphate. PA1 (a) conversion of the 6-keto group into a 6-amino group, PA1 (b) conversion of the 2-amino group to a 2-fluoro group, and PA1 (c) conversion of the ribofuranosyl moiety to an arabinofuranosyl moiety. Steps (a), (b), and (c) can be performed individually or concomitantly and in any sequence. PA1 (a) conversion of the ribofuranosyl sugar moiety to the arabinofuranosyl moiety via an intermediate having a 3',5'-disiloxane bridge, a 2'-oxo group and a 2-fluoro group; PA1 (b) production and use of 3',5'-diacyl-2'-OSO.sub.2 R.sup.2 -2-fluoro intermediate during sugar conversion; PA1 (c) production and use of 3',5'-diacetyl-2'-oxo-2-fluoroadenine intermediates during sugar conversion; PA1 (d) halogen exchange converting 6-chloro or 6-bromo compounds to 6-fluoro, utilizing DABCO as a catalyst; PA1 (e) acylation utilizing an HF-pyridine medium from a prior fluorination step; and PA1 (f) acylation of 2-amino-adenosine compounds with subsequent fluorination.
One of the disadvantages of this process is that the protected sugar (TBNA) used in step (b) is very costly and has limited commercial availability. Since the diazotization/fluorination reaction in step (d) has a relatively low yield, the overall yield of fludarabine or fludarabine phosphate from this costly material is undesirably low. Therefore, it would be preferable to provide a process which avoids the use of the costly protected sugar. In addition, since the coupling reaction may be difficult to scale-up, and is not completely stereoselective, it would be desirable to have a process which does not require a coupling step of this type.
Although the yield and reliability of the coupling reaction are improved and the reaction time is decreased substantially by using the modified procedure described in WO 91/08215, this modified procedure requires the use of a more complex anhydride, which is not commercially available. This adds an additional step to the process, because the anhydride itself must be prepared.
The 2,6-diaminopurine starting material also is expensive and has limited commercial availability. Therefore, it would be desirable to provide a process which does not require this starting material.
A further disadvantage of the prior art process, as indicated above, is the low yields of 2-fluoro-adenosine compound from the diazotization/fluorination step using the THF-HBF.sub.4 system. Therefore, it would be advantageous to provide a process wherein the 2-fluoro group is introduced more efficiently.